PGR and breast cancer: Triple-negative breast cancer (TNBC), one subtype of breast cancer with a negative expression profile for the three proteins, estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor type 2 (HER2), constitutes about 15% of breast cancer cases [4] and has high risk of metastasis [5], particularly to the visceral organs, such as the lung, liver, and brain.